The disclosed invention relates generally to mixers and more particularly to overload protection for mixers. A mixer is used for frequency conversion or phase detection in such applications as spectrum analyzers, selective level meters, phase-locked loops, and radio receivers. One common form of a double-balanced mixer is shown in FIG. 1. The mixer shown there utilizes a diode bridge 10 consisting of four diodes 11-14 connected in series to form a loop. A pair of transformers 17 and 111 are connected to diode bridge 10 to enable input signals from a pair of input sources such as local oscillator 15 and radio frequency source 19 to be applied to the diode bridge. Nodes B and D of the diode bridge are connected to opposite ends of the secondary winding of transformer 17 and the center-tap of this secondary winding is connected to ground. Similarly, nodes A and C of diode bridge 10 are connected to opposite ends of the secondary winding of transformer 111. An output signal is produced at the center-tap of the secondary winding of transformer 111 and is represented in FIG. 1 as the signal IF between this center-tap and ground.
When this mixer is utilized for frequency conversion, the amplitude of the local oscillator input signal is typically selected to be much larger than the amplitude of the radio frequency input signal. Under these conditions, the voltage drops across diodes 11-14 are dominated by the voltage applied to the diode loop by transformer 17. Because of the grounded center-tap of the secondary of transformer 17, the local oscillator tends to drive the voltage of node B to minus the voltage of node D. Therefore, when node B is driven high, diodes 11 and 12 are reverse biased and diodes 13 and 14 are forward biased. This results in a pair of high impedance paths from node A through diodes 11 and 12 to the grounded center-tap of the secondary winding of transformer 17 and also results in a pair of low impedance paths from node C through diodes 13 and 14 to the center-tap of the secondary winding of transformer 17. In effect, the diodes 11-14 act as switches which are individually placed into either their high impedance states or their low impedance states in response to the local oscillator signal.
The diodes 13 and 14 are matched so that under the forward current induced by the local oscillator input signal these two diodes have substantially equal impedance. For such matched impedances and for zero radio frequency input signal, node C will be at ground potential. For a small radio frequency input signal compared to the local oscillator input signal, the voltage of node C will remain substantially at ground potential. However, for such small radio frequency input signals, the large reverse biased impedance of diodes 11 and 12 will result in larger swings in the voltage of node A. In effect, the bottom of the secondary winding of transformer 111 is held at ground potential while the top of this winding is allowed to float (i.e. the large reverse biased impedance of diodes 11 and 12 effectively decouple the top of this winding from nodes B and D). Therefore, the output signal IF equals the signal produced in the lower half of the winding due to the radio frequency input signal.
When the local oscillator input signal swings low, diodes 11 and 12 become forward bi sed and diodes 13 and 14 become reverse biased. Diodes 11 and 12 are matched so that they have the same forward biased impedance under operating conditions. By the same reasoning used above, it can be seen that now the top of the secondary winding of transformer 111 is held effectively at ground potential and the bottom of that winding is allowed to float. The effect of this is that polarity of the output signal IF reverses each time the polarity of the local oscillator input signal reverses. The output signal is therefore proportional to the product of the radio frequency signal and a square wave of alternating voltages of equal amplitude and opposite polarity. The output signal therefore contains frequencies equal to the frequency of the radio frequency input signal plus or minus integral multiples of the frequency of the local oscillator input signal. In a real mixer there are deviations from the operation discussed above which result in the output signal containing frequencies equal to integral multiples of the frequency of the radio frequency input signal plus or minus integral multiples of the frequency of the local oscillator input signal.
The local oscillator input signal is typically selected to be large enough that the current produced through the forward biased diodes is near the maximum forward biased current which the diodes can handle without being damaged. This is done so that the forward biased impedance is small in order to substantially ground the node between the two forward biased diodes (Note that since the radio frequency input signal is small, the relevant impedance for evaluating the effect of this signal is the differential impedance equal to the derivative of voltage across a diode with respect to the current through the diode--namely dV/dI which is inversely proportional to the current I). Unfortunately, this choice of operating conditions makes these diodes particularly susceptible to damage by input signals larger than rated operating input signals.
In many applications, the mixer is located at an input port of a device so that the local oscillator input signal and/or the radio frequency input signal are supplied by the user. Such a mixer therefore requires protection from input signals which exceed the rated range of the mixer. The primary existing ways to protect against excess power which might damage the diodes are either to tell the user the maximum power allowed and rely on his adherence to such advice (an obviously risky approach) or to place amplifiers or pads ahead of the mixer with built-in overload protection. This overload protection could be in the form of voltage clamps to limit the maximum input voltage to some predetermined value. Another protection scheme is shown in the circuit presented in FIG. 2. In that scheme, each leg of the diode bridge 20 includes a pair of diodes so that the power dropped in any given leg of the diode bridge is split between the two diodes in that pair thereby enabling the diode bridge to handle twice the power without damage. Unfortunately, if this diode bridge has similar operating characteristics to the circuit in FIG. 1, then it should carry the same forward current as the diode bridge in FIG. 1 and therefore requires twice the input power required by the mixer in FIG. 1 and is susceptible to damage by the same percentage increase in power as for the circuit in FIG. 1.